1. Field of the Invention
The present invention relates to an ultrasound imaging apparatus that transmits ultrasound waves to a subject and generates an ultrasound image based on the waves reflected from the subject, more specifically, relates to a transmission system that transmits ultrasound waves.
Moreover, the present invention relates to a method for generating an ultrasound image.
2. Description of the Related Art
For an ultrasound imaging apparatus, an ultrasound transmission/reception circuit has been developed for the purpose of increasing the resolution and sensitivity of an ultrasound image. An arbitrary waveform transmission circuit that is capable of transmitting various waves in accordance with imaging modes has been developed, which is capable of not only transmitting rectangular pulse waves but also transmitting sine waves subjected to Gaussian amplitude modulation so that second harmonic waves are suppressed, transmitting multi-frequency waves in which a plurality of frequencies are combined, and transmitting chirp waves.
As a conventional technique, an ultrasound imaging apparatus provided with a linear amplifier is proposed (e.g., Japanese Unexamined Patent Publication No. 2000-152930). A linear amplifier is capable of securing linearity and operating at high speeds when a bias current is made to flow. An arbitrary waveform transmission circuit of a conventional technique will be described with reference to FIG. 1. FIG. 1 is a block diagram showing a transmission circuit.
The transmission circuit shown in FIG. 1 configures a complementary amplifier. This transmission circuit reversely operates.
When an input voltage Vin is inputted, positive and negative signals are transmitted from input-stage transistors M1 and M2, respectively, and amplified in amplifier-stage transistors M5 to M8 via bias gate transistors M3 and M4. The amplified signals are supplied to array transducer elements 2 from buffer-stage transistors M9 and M10.
Further, this transmission circuit is provided with a part for controlling a bias current. By operating with two power sources VP and VN, this transmission circuit is also provided with two types of bias control power sources. To the gates of the bias gate transistors M3 and M4, control power sources CS1 and CS2 that generate voltages VG1 and VG2, respectively, are connected.
The voltages VG1 and VG2 are controlled so that a bias current flows during a waveform transmission period (a time when a waveform is outputted and predetermined times before and after the time). Since this transmission circuit is bipolar, the bias gate transistor M3 is n-channel and the bias gate transistor M4 is p-channel. Moreover, as the voltages VG1 and VG2, opposite voltages are given.
The operation sequence of the abovementioned transmission circuit is shown in FIG. 2. FIG. 2 is a timing chart showing the operation sequence of the transmission circuit of the conventional technique. The timing chart of FIG. 2 shows the generation voltage waveforms of the control power sources CS1 and CS2 and the waveforms of the bias current, the input voltage Vin and an output voltage Vout. In the abovementioned transmission circuit, the voltages VG1 and VG2 of the control power sources CS1 and CS2 are switched between High and Low so that the bias current flows only when a transmission signal is outputted. Thus, the bias current does not flow when a transmission signal is not outputted.
To be specific, the voltages VG1 and VG2 of the control power sources CS1 and CS2 are switched between High and Low so that supply of the bias current is started a predetermined time before a transmission signal is outputted. Then, the voltages VG1 and VG2 of the control power sources CS1 and CS2 are switched between High and Low so that the supply of the bias current is halted after a lapse of a predetermined time after an arbitrary waveform is transmitted.
This transmission circuit is brought into the active state when a bias current is supplied, thereby being capable of amplifying an input signal. However, in a case that an electric current continuously flows at all times, the power consumption of the transmission circuit increases, and therefore, the temperature of the circuit increases. The ultrasound imaging apparatus executes pulse transmission of ultrasound waves to a living body at predetermined time intervals and thereby receives the waves reflected from an arbitrary depth. Since the transmission circuit is in the halted state except a time when a transmission signal is outputted, the operation of the transmission circuit is halted by halting the supply of the bias current. Thus, increase of the power consumption of the transmission circuit is suppressed, and consequently, it is possible to suppress increase of the temperature of the transmission circuit.
However, in the abovementioned transmission circuit, the bias current is radically turned on/off. Therefore, as shown in FIG. 3, glitch noise is generated when the bias current is turned on/off. The apparatus normally operates to detect the waves reflected from the structure of a living body of the transmission pulses. However, as shown in FIG. 3, different transmission pulses (glitch noise) exist before or after the transmission waveform, and therefore, an ultrasound image may be duplicated. Moreover, there is the fear that the frequency of a transmission band different from a transmission band to image may be detected and consequently resolution may decrease.
In particular, in a case that a transmission waveform is outputted at a low transmission voltage, for example, in the contrast imaging mode of administering an ultrasound contrast agent into the vessel and imaging without destroying the ultrasound contrast agent, glitch noise may reach a voltage level that cannot be ignored with respect to the transmission voltage. In this case, there is a problem that the glitch noise conspicuously appears as an artifact in an ultrasound image. For example, in a case that the voltage of a transmission signal is about several volts, glitch noise reaches a voltage level that cannot be ignored with respect to the transmission signal.